What is the X in X-mas?
isotone i·so·tone (ī'sə-tōn')
One of two or more atoms whose nuclei have the same number of neutrons but different numbers of protons.
any of two or more species of atoms or nuclei that have the same number of neutrons. Thus, chlorine-37 and potassium-39 are isotones, because the nucleus of this species of chlorine consists of 17 protons and 20 neutrons, whereas the nucleus of this species of potassium contains 19 protons and 20 neutrons.